Two conventional microanalysis techniques for determining elemental constituents in a specimen involve detecting characteristic X-rays that are produced by bombarding the specimen with high-energy electrons. Both techniques generate a spectrum in which peaks correspond to specific X-ray lines and from which the elements can be easily identified. In a first technique called “energy-dispersive spectroscopy” (EDS), the characteristic X-rays are detected with an energy-dispersive spectrometer, which is solid-state device that discriminates among X-ray energies to produce a complete spectrum. In a second technique, called “wavelength-dispersive spectroscopy” (WDS), the characteristic X-rays are detected with a crystal spectrometer that uses a diffracting crystal to select a wavelength of interest which is then detected. The complete spectrum is acquired sequentially as the full wavelength range is scanned.
Detectors for EDS consist of cooled Si(Li) detectors, HPGe detectors and, more recently, silicon drift detectors. However, all these detectors have limited count rates and dynamic ranges.
In WDS applications, for many decades, photomultiplier tube (PMT) and gas-filled proportional counters have been the preferred X-ray detectors; due to their acceptable quantum efficiency, adequate dead time, and good proportionality. In gas-filled detectors, various ionizing gases such as xenon, argon, neon, and P-10 are commonly used to provide the best elemental sensitivity and are contained in a closed detector housing having an entrance window to permit entry of the X-rays. These detectors have some drawbacks. In particular, elements whose atomic number is less than eleven are considered “light” in X-ray microanalysis. These light elements produce “soft” or low energy X-rays that are hard to detect because they are easily absorbed. Therefore, detectors for X-rays generated from light elements require thin and delicate windows to admit the X-rays without substantial absorption. These thin windows are commonly made of beryllium, Mylar, polypropylene and other materials where the materials and thicknesses are chosen to minimize the absorption of x-rays. Thin materials require precise techniques for gluing the window material to the detector housing in order to minimize the number of microscopic leaks that allow the ionizing gas to escape from the housing. Even with the best of such techniques, detectors employing ultra thin windows require a constant supply of ionizing gas due to leakage. Further, ultra thin windows can also break and require replacement.
In addition, gas proportional counters saturate between 75,000 to 125,000 counts per second. Any element with a short crystal-to-sample distance is prone to saturation if present in high concentrations. Some common examples are Al and Si on a TAP crystal; Ti, Cr, V, and Mn on a PET crystal; and Ni, Cu, Zn on a LiF crystal. The low saturation limit often requires the incident electron beam current to be increased in order to analyze trace elements more quickly and then reduced in order to prevent saturation. Automated systems which switch between high current and low current beam conditions can be used but require additional time to generate the X-ray spectrum. The same count rate limitation is observed also for x-ray fluorescence (element micro analysis) and in EDS.